Fix a couple of deprecated things.

[ghost]

Fixes #1101

• ctx.begin() is deprecated in favor of ctx.out()
• fmt/time.h is deprecated in favor of fmt/chrono.h
• The library is compiled with C++11, but auto deduced return types is a C++14 feature. This is only in the docs, but we should be consistent

I agree that my contributions are licensed under the {fmt} license, and agree to future changes to the licensing.

[vitaut]

Merged in b588d7f with minor tweaks (we shouldn't be limited to C++11 in examples, the library has stricter portability requirements than most of the user code). Thanks!